I think it's supposed to help their digestive process which means their immune systems are stronger (better able to cope with worms themselves, for example). But really not sure - that seems a fairly dodgy explanation looking back at it. You can only use it in plastic drinkers and I've got a metal one for the hens now - plus we have lots of other water around for the ducks and they muddy it all up so it has to be emptied out daily (in other words, I'd have emptied the cider vinegar in a day).
H